Understanding how individual cells and complex brain networks interact in both time and space has been one of the grand challenges in the 21st century. In 2010, Lee et. al. have demonstrated that optogenetic fMRI (ofMRI) within the living mammalian brain reveals BOLD signals in downstream targets distant from the stimulation site, indicating that this approach can be used to map the global effects of controlling a local cell-type specific neuronal population. Since then, multiple studies have utilized ofMRI to dissect brain networks and properties. In this session, technical considerations in the application of ofMRI will be examined. Studies dissecting brain networks and properties using ofMRI will be reviewed. The opportunities and challenges will be discussed.
Target Audience
Neuroimaging clinicians, scientists and engineers who are interested in neuromodulation, optogenetic, brain networks, brain functions and pre-clinical imagingHighlights
Objectives
Purpose
The brain is a highly interconnected structure with parallel and hierarchical networks distributed within and between neural systems (Oh et al., 2014; Stern, 2013). This integrative architecture dictates the underlying principles of how brain-wide neural connectivity supports and organizes sensory, behavioral and cognitive processes (Park and Friston, 2013). Resting-state functional MRI (rsfMRI) connectivity non-invasively maps brain functional networks using slow and spontaneous hemodynamic fluctuations (Fox and Raichle, 2007). However, dissecting the brain networks remain challenging due to the correlative nature of rsfMRI. Hence, there is a need to utilize direct neuromodulation techniques to investigate the spatiotemporal properties of brain-wide large-scale neural interactions. The neuronal architecture within the brain is an intricate web of interconnected excitatory neurons and inhibitory interneurons. This creates a highly dynamic yet stable excitation-inhibition network. Normal operation of brain-wide networks requires precise and spatiotemporally structured activity propagation and interaction patterns to support the functional properties of these networks.
Electrical stimulation cannot target cell-type specific neurons as it excites circuit components in the stimulation region that may not be involved in the particular behavior. Hence, this technique will not likely permit a reliable probe of the spatiotemporal activity propagation dynamics and their functional output. Furthermore, electrode recording site density and spatial coverage limit the characterization of properties in brain-wide large-scale brain functional networks. In 2010, Lee et. al. have demonstrated that optogenetic fMRI (ofMRI) within the living mammalian brain reveals BOLD signals in downstream targets distant from the stimulation site, indicating that this approach can be used to map the global effects of controlling a local cell-type specific neuronal population (Lee et al., 2010). This method can overcome the limitations of electrical stimulation non-specificity and electrode recording spatial coverage in characterizing large-scale brain-wide neural activity propagation and interaction dynamics. In this session, technical considerations in the application of ofMRI will be examined. Subsequently, studies dissecting brain networks and properties using ofMRI will be reviewed. Lastly, the opportunities and challenges will be discussed.
Method
Optogenetics (Boyden et al., 2005) is a neuromodulation method that uses a combination of techniques from optics and genetics to achieve reversible and millisecond control of specific neuronal population. This technique involves the following basic steps. First, a gene encoding opsin which is a light-sensitive protein such as ChR2 and a promoter which controls the expression such as CaMKIIa are packed into a viral vector. When the virus is injected into the brain of the animal, it infects the cell body of neurons and delivers the construct. After a period of time, usually six weeks for ofMRI, the opsins are only expressed in the membrane of a subgroup of cells. These cells can then be triggered with light of a specific wavelength, typically delivered through an optic fiber through the skull of the animal. Upon optogenetic stimulation, fMRI can be utilized to monitor the brain-wide responses.Technical Considerations
Previous studies have demonstrated that high light power density (> 150 mW/mm2) delivered to a naïve brain can result in pseudo positive and negative BOLD responses at the stimulation site (Christie et al., 2013; Schmid et al., 2016). To minimize such artifacts, low light power density should be used. Note that fMRI may not be sensitive enough to detect weak responses resulted from low light power density optogenetic stimulation. Strong expression of the opsins, and identical viral injection site and fiber implantation site can produce robust optogenetic responses. Undesired visual stimulation to the animals during optogenetic stimulation has been reported previously (Schmid et al., 2016). To avoid such undesired visual stimulation, one can make the exposed optical fiber cannula opaque using heat shrinkable sleeves.Dissecting Brain Networks and Properties using ofMRI
To determine the spatiotemporal response properties of long-range excitatory thalamic projections to cortical and subcortical regions, ofMRI was applied to interrogate the vibrissae (whisker) somatosensory thalamo-cortical system (Leong et al., 2016). Low (1 Hz) frequency optogenetic stimulation of the somatosensory thalamus (ventral posteromedial) evoked large-scale BOLD fMRI activation in bilateral primary visual, somatosensory, and auditory cortices, but not high frequencies (5–40 Hz). Enhanced visually evoked fMRI activation in the superior colliculus was also observed during and after low frequency stimulation at the somatosensory thalamus, indicating that visual processing was subcortically modulated by low-frequency activity originating from the somatosensory thalamus.
Large-scale spatiotemporal specific hippocampal-cortical activity was also examined using ofMRI (Chan et al., 2016). Low (1 Hz) frequency optogenetic stimulation of dDG excitatory neurons evoked robust cortical and subcortical responses including bilateral primary visual cortex, secondary visual cortex, lateral geniculate nucleus and superior colliculus, as well as cingulate cortex. In contrast, high (40 Hz) frequency optogenetic stimulation evoked strong positive responses in the bilateral dorsal hippocampus, and negative responses in bilateral ventral hippocampus. Furthermore, the interhemispheric rsfMRI connectivities of the hippocampal, visual, auditory and somatosensory networks were enhanced during and after low frequency optogenetic stimulation. These findings indicate that low frequency activity propagates in hippocampal-cortical pathway to drive brain-wide rsfMRI connectivity.
Opportunities and Challenges
Current optogenetic tools include activators and silencers of different cell types such as excitatory neurons and inhibitory interneurons (Yizhar et al., 2011). Most of the ofMRI studies utilized activators to modulate excitatory neurons and inhibitory interneurons. However, the effects of silencing excitatory neurons or inhibitory interneurons to brain-wide networks and properties remain largely unexplored. Furthermore, limited studies have applied simultaneous multi-site neuromodulation in examining the interactions between neural systems. The combination of different optogenetic perturbation strategies with large field-of-view fMRI may provide further insights in dissecting brain networks and properties.
The aforementioned studies utilized different frequencies of optogenetic stimulation and fMRI to dissect brain networks and properties. However, the ideal optogenetic stimulation has yet to be established (Hausser, 2014). First, the level of stimulation may drive neural responses outside the physiological range. Second, light stimulation and optogenetic expression are not uniform across the target neural population, generating heterogeneity in magnitude and spatial extent of optogenetic manipulation. Third, stimulation synchronizes the target neural population, potentially driving the circuit into unphysiological patterns of activity. Future studies may encompass a more ideal and physiological stimulation such as using a fiber bundle for patterned asynchronous stimulation.
I thank Prof. E. X. Wu and Mr. A. T. Leong for the insightful comments.
This work was supported by the Hong Kong Research Grant Council (HKU17103015), State Key Laboratory of Pharmaceutical Biotechnology at the University of Hong Kong, and a generous donation from the Lam Woo Foundation.
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Other ofMRI studies dissecting brain networks and properties:
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Chan, R.W., Leong, A.T.L., Fan, S.J., Gao, P.P., Chan, Y.S., Yung, W.H., Tsia, K.K., Wu, E.X., 2015b. Low Frequency Stimulation of Hippocampus Evoked Large-scale Cortical and Subcortical Responses: an Optogenetic Functional MRI Study. 2015 proceedings of Society for Neuroscience (Oct 17-21, Chicago), 449.419.
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Lee, H.J., Weitz, A.J., Bernal-Casas, D., Duffy, B.A., Choy, M., Kravitz, A.V., Kreitzer, A.C., Lee, J.H., 2016. Activation of Direct and Indirect Pathway Medium Spiny Neurons Drives Distinct Brain-wide Responses. Neuron 91, 412-424.
Leong, A.T.L., Chan, R.W., Gao, P.P., Cheng, J.S., Zhang, J.W., Fan, S.J., Tsia, K.K., Wong, K.K., Wu, E.X., 2015a. Frequency specific optogenetic recruitment of evoked responses in the somatosensory thalamocortical circuit. 2015 proceedings of International Society for Magnetic Resonance in Medicine (May 30 - June 5, Toronto), pp135.
Leong, A.T.L., Chan, R.W., Gao, P.P., Fan, S.J., Tsia, K.K., Chan, Y.S., Yung, W.H., Wu, E.X., 2015b. Optogenetic fMRI Investigation of the Spatiotemporal Characteristics of the Ventral Posteromedial Thalamus (VPM). 2015 proceedings of Society for Neuroscience (Oct 17-21, Chicago), 449.418.
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Other ofMRI studies:
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